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Take Quadratic And Cubic, Get Sum And Product (Posted on 2007-08-24) Difficulty: 3 of 5
Determine all possible positive integer triplets (x, y, z) with y ≥ z satisfying the following system of equations :

                  x2 = 2(x+y+z)

              x3- y3 - z3 = 3xyz

  Submitted by K Sengupta    
Rating: 3.8000 (5 votes)
Solution: (Hide)
From the second equation, we observe that x> y, z.

Accordingly, from the first equation, we obtain x^2 < 6x, so that :
x< 6.

The RHS of the first equation is even, so that x = 2, 4.

Suppose x=2. If so, we must have y+z = 0 in terms of the first equation.. This does not admit of any positive integral solution which is a contradiction.

Suppose x =4. Then the first equation yields y+z = 4. Imposing the restriction y>=z, this has the solutions (y, z) = (3, 1); (2, 2)

Thus, (x, y, z) = (4, 3, 1); (4,2, 2) correspond to all possible solution to the given problem.

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Praneeth provides a solution to the general case of determining all possible integer triplets that satisfy conditions of the problem in this location.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
SolutionAlternative approachFrankM2008-01-19 21:51:13
re(2): Solution (little thing to add)Dej Mar2007-08-25 09:11:05
re: Solution (little thing to add)Daniel2007-08-25 02:15:58
SolutionSolutionPraneeth2007-08-24 15:23:57
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